Many loads connected to AC supplies require DC power and convert the AC power into DC power.
This is common, for example, in aircraft in which the aircraft is equipped with an internal 3-phase network. The frequency of the electric current over the power supply network can be variable.
In order to reliably power a load requiring a DC supply, an AC to DC converter is provided between the power supply network and the load.
It is necessary, particularly in aircraft to avoid or reduce harmonic currents generated by the conversion process and several solutions are available. Such solutions include passive filtering combined with 6-pulse rectification, active rectification, phase multiplication and the like. In particular, aircraft use multiphase conversion as this is known to be particularly reliable.
Aircraft AC to DC conversion systems often use a multiphase transformer, supplied from a 3-phase source and generating a multiphase AC supply for conversion into DC power.
Systems are known using a 12-pulse ATRU arrangement in which two rectifier converters are arranged in parallel.
As shown in FIG. 1, a 12-pulse ATRU comprises a primary winding 5 in a delta configuration and two secondary windings 7 in a star configuration. The primary winding 5 comprises windings 5a, 5b and 5c, while the two secondary windings 7 comprise windings 7a-7f. 
The two secondary windings 7 are connected to two rectifier bridges 11 and 13 which operate in parallel and, to balance DC currents and avoid circulating currents, two interphase transformers 15 and 17 are required.
The ATRU 3 converts a 3-phase supply, e.g. the power supply on board an aircraft, as applied to conductors 9a, 9b and 9c into a first sub-supply 19 comprising conductors 19a, 19b and 19c and a second sub-supply 21 comprising conductors 21a, 21b and 21c. The sub-supplies are rectified by rectifier bridges 11 and 13. The outputs 11a, 11b, 13a and 13b from the rectifier bridges 11 and 13 are combined by the two interphase transformers or chokes 15 and 17. The converter provides a DC output voltage.
12-pulse rectifiers, commonly used in aircraft and aerospace applications are known to generate some total harmonic distortion.
In modem aircraft especially those with electric power systems operating over a wide frequency range but also with constant frequency systems, it is desirable to keep the level of total harmonic distortion low, and sometimes below that generally achievable with 12-pulse systems. An 18-pulse or more system can reduce harmonic distortion, providing a system with better power quality. An example of an 18-pulse converter can be found in U.S. Pat. No. 8,729,844.
Known ATRUs typically require the use of dedicated DC link chokes or inductors to smooth the output current, and thus contribute to reduction of harmonic distortion at the input, in order to meet required power quality standards. FIG. 2 shows an 18-pulse ATRU converter including dedicated DC link chokes (L5) connected in series with interphase power transformers (IPT1, IPT2). The rectified output provided by the ATRU is provided to the DC link filter 122 which includes a DC differential mode inductor L5 and capacitor C10 for smoothing the rectified output generated by the ATRU. This further reduces harmonics.
The DC link choke contributes to the total weight, size and the cost of the motor drive system.
The aim of the present disclosure is to reduce or eliminate the need for a dedicated DC link choke for 18 pulse ATRU, and ATRUs with higher number of pulses, without compromising reliability or cost.